In electrical circuits and measurement circuits in particular, it is often necessary or desireable to develop a voltage proportional to an electrical current or alternating current voltage. Transformers are widely known and used for such a purpose. Transformers are well-known to achieve this function by using a current to induce a magnetic flux in a core of iron or other low reluctance material and to use that flux to induce a voltage in another conductor. The principles of such a transformer have long been well understood. For measurement or other applications requiring extreme accuracy or efficiency in such a conversion, saturation of the core, eddy current losses, frequency effects and the like must be closely controlled. Where high accuracy is concerned, core saturation is recognized to be a major problem. Also, in measurement circuits, extreme care must be taken to minimize the effects of the measurement being conducted on the operation of the circuit itself.
This latter consideration is of particular concern in the measurement of large currents where it is often desireable or necessary to divide the current into proportional fractions by means of proportional resistances and to directly measure the smaller fraction thereof, inferring the total current therefrom. However, as the current becomes larger, it is then necessary to divide the current into more disparate fractions which, in turn, tends to increase the effect of the measurement on the quantity measured, thereby decreasing the accuracy of the measurement. For example, in an arrangement such as that disclosed in U.S. Pat. No. 4,182,982 to Wolf et al, an apertured conductor is provided with a core extending through the aperture and surrounding one leg of the divided current path around the aperture. However, any current induced in the secondary winding wound around the core will cause an E. M. F. opposing the flux in the core and reduce the current flow through that leg of the divided current conductor. This effect will be aggravated as the proportionality of the resistances of the respective legs of the divided conductor is increased. Such an arrangement also makes no provision for controlling flux in the magnetic core and core saturation by the induced magnetic flux may also cause errors in the measurement and, in any event, limits the range of currents over which such a transducer is usable.
The arrangement of Wolf et al also makes no provision for control of resistivity changes in the respective legs of the current divider which may be due to temperature and, therefore, the device is subject to errors induced by changes or gradients in ambient temperature and, often more importantly, by changes due to resistive heating of the current divider arrangement itself or even heating from eddy current losses in the core.
The problem of core saturation is commonly approached by arranging for at least a portion of the induced magnetic flux in the core to be cancelled. This is often done by providing an auxiliary winding on the core and a calibrated current source to cancel predetermined fixed portions of the flux in the core. Such an arrangement is, at best, expensive and requires substantial and difficult design considerations as well as introducing multiple sources of measurement error since the impedance of any current supply circuit must have a finite value and coupling efficiency of such an auxiliary coil will be less than perfect.
Accordingly, in some applications, passive flux cancelling arrangements have been used. For instance, in U.S. Pat. No. 4,626,778 to Friedl, a current is coupled to two separate conductors which are identical in plan view but of differing thickness and, hence, of differing cross-sectional areas and differing resistances. These conductors link the cores with currents arranged in opposing directions to provide flux cancellation for limitation of core saturation. The structure proposed by FriedL is singularly inappropriate for use at high currents or for accurate measurement thereof. Specifically, the arrangement of FriedL requires that connections be made to a plurality of conductors; which connections are subject to aging. Also, while the conductors are placed in close proximity to each other, no provision is made to insure that resistance changes with temperature of the conductors, particularly due to potential resistive heating, have limited effect on the proportionality of the flux in the core to the current traversing the split windings. Further, the resistance of connections to each of the split, one-turn windings is not negligible and, moreover such resistance is subject to change by aging and thermal cycling and is inherently not easily susceptible of accurate calibration.